1. Technical Field
The present disclosure relates to a microphone, and more particularly, to a microphone with a specific audible area using ultrasound wave, which emits an ultrasound wave toward an audible sound source positioned in a specific area within a desired distance and a desired direction, using an ultrasound transducer, and extracts a sound signal in an audible frequency range, generated by the audible sound source, from an ultrasound wave reflected from the audible sound source, such that a user can selectively hear a desired sound even in a noisy environment. Hereafter, the audible sound source is referred to as sound source.
2. Related Art
Since an ultrasound wave is a sound wave with a higher frequency than a sound wave in the audible frequency range (hereafter, referred to as audible sound wave), they share many properties. Both ultrasound and audible sound waves have the same propagation velocity, and experience the same non-linear interaction while two sound waves are propagating through the same propagation path in the same propagation direction. The difference is that the ultrasound wave has a much shorter wavelength than the audible sound wave. Because of this wavelength difference, the ultrasound wave has an excellent going-straight property or propagates only in a predetermined direction compared to the audible sound wave. Thus, as energy is concentrated only in the predetermined direction during wave propagation, ultrasound wave can be focused toward the predetermined direction. Based on the non-linear interaction between two ultrasound waves propagating in the same direction, a directional speaker has been developed.
That is, one ultrasound wave with a certain center frequency is modulated with an audible sound signal and the other ultrasound wave with the same center frequency is not modulated, and then both modulated and unmodulated ultrasound waves with the same center frequency are transmitted in a specific direction, a user at a far distance in the corresponding direction can hear the original audible sound with his/her ears due to the non-linear interaction of two ultrasound waves
However, this technology can be applied only to a speaker, but cannot be applied to a microphone. Thus, when a user intends to remove the influence of surrounding noise in a noisy environment and to selectively hear only desired sound, the technology cannot be applied.